The present invention generally relates to white balance adjusting apparatuses, and more particularly to a white balance adjusting apparatus in which a white balance correction control circuit is controlled depending on a color temperature detected by color temperature sensors so as not to carry out an erroneous white balance adjustment even when the illuminance is extremely high or low.
In a video camera using a conventional white balance adjusting apparatus, light from an object which is to be picked up reaches an image pickup device via an optical system. A signal which is obtained in the image pickup device by a photoelectric conversion is supplied to a YRB separating circuit wherein the signal is subjected to signal processings such as amplification, color demodulation and gamma correction. As a result, a luminance signal Y and red and blue primary color signals R and B are produced from the YRB separating circuit, and the primary color signals R and B are respectively supplied to first and second amplifiers.
On the other hand, a sensor part is provided in a vicinity of the optical system on a front of the video camera. A diffusion plate is arranged in front of the sensor part, and a pair of color temperature sensors receive light from the object and the surroundings which is integrated by the diffusion plate. The color temperature sensors have mutually different spectral characteristics and have a high sensitivity with respect to red and blue, respectively. Output signals of the color temperature sensors are supplied to a color temperature detecting circuit which supplies to a white balance correction control circuit a color temperature detection signal having a level dependent on the detected color temperature. The white balance correction control circuit supplies to the first and second amplifiers control signals having levels dependent on the color temperature detection signal, that is, control signals having levels which correspond 1:1 to the detected color temperature. The gains of the first and second amplifiers are controlled responsive to the control signals so that the gain of the primary color signal B is made large and the gain of the primary color signal R is made small when the color temperature is low, for example.
The primary color signals R and B from the first and second amplifiers and the luminance signal Y from the YRB separating circuit are supplied to a video signal processing circuit which produces a standard system color video signal from these signals. Accordingly, the white balance is adjusted by controlling the gains of the first and second amplifiers by the control signals.
Photodiodes which produce signals having levels approximately proportional to the illuminance are generally used for the color temperature sensors. In addition, the color temperature detecting circuit generally comprises first and second logarithmic amplifiers and a differential amplifier, and the output color temperature detection signal of the color temperature detecting circuit has a level log(La/Lb) when the output signals levels of the color temperature sensors are denoted by La and Lb.
However, the color temperature sensors become saturated when the illuminance is extremely high, and the logarithmic relation cannot be maintained between the input and output signal levels at each of the first and second logarithmic amplifiers when the illuminance is extremely low and the output signal levels La and Lb of the color temperature sensors are extremely small. For this reason, there is a problem in that an erroneous white balance adjustment is carried out when the illuminance is extremely high or low.